The use of inductor capacitor (LC) resonant frequency circuits is well known for applications which include electronic article surveillance (EAS), chip based radio frequency identification (RFID), chipless RFID and other such applications. In such applications there are three key circuit parameters, which are typically employed for quantifying the electrical performance of the circuit, and, in particular, the antenna portion of the circuit. The three parameters are (1) the center frequency of the resonant circuit, (2) the quality factor (Q factor) of the resonant circuit, and (3) the relative output signal amplitude of the resonant circuit. With such circuits, the bandwidth is defined as the difference between an upper frequency and a lower frequency (F2) of the circuit at which the output amplitude response is 3 dB below the passband response. The output signal amplitude is a measured height of response of the circuit based on a fixed position and a fixed incident magnetic field strength. The quality or Q factor is the ratio of the center frequency of the resonant circuit divided by the bandwidth output signal of the circuit.
LC resonant frequency circuits are well known in the art. When used for EAS, such circuits are formed into labels or tags which are applied to goods to be protected. As an example, a tag may be formed of a dielectric substrate having first and second generally parallel planar surfaces on opposite sides thereof. A first side of the substrate includes a first conductive pattern in the form of a coil (forming the inductor of the circuit), a first end of which terminates in a generally square or rectangular plate forming a first electrode of the capacitor portion of the circuit. The second surface of the substrate includes a second generally square or rectangular plate forming the second electrode of the capacitor portion of the circuit and a conductive trace extending away from the capacitor plate to a point proximate an edge of the substrate. The distal end of the conductive trace is electrically connected by a weld through or around the edge of the substrate to the second end of the coil to thereby complete the parallel LC circuit. When a tag of this type is exposed to electromagnetic energy at or near the center frequency of the tag, as determined by the values of the inductor and capacitor in accordance with a known formula, the circuit resonates.
FIG. 1 shows four typical output signals resulting from the resonance of four respective typical LC tag circuits. A first one of the output signals 2 shown in FIG. 1 is marked to show the center frequency (Fc), the upper frequency (F1) and the lower frequency (F2) of the respective LC tag having the output signal. Of course, the difference between the upper frequency and the lower frequency establishes the bandwidth. A second one of the output signals 4 shown in FIG. 1 is also marked to show the amplitude (A1) of the respective output signal from the tag described above.
The present invention seeks to improve the performance of a typical LC resonant circuit. In particular, the present invention is aimed at enhancing or amplifying the output signal amplitude response from an LC resonant circuit.